The present invention relates to a circuit arrangement, having at least one electric main switch with a reference electrode, a control electrode and a work electrode, and a free-wheeling diode which is connected antiparallel to the main flow direction of each main switch.
In electronics in general but especially in power electronics, there is a need to be able to process the highest possible power levels while occupying the least possible space. This is associated with the demand for keeping incident power losses minimal, since a power loss is converted into heat and as a result, (larger) cooling bodies and hence larger housing dimensions are needed. In externally-controlled and self-controlled transistor inverters, for instance, the incident power loss is less, the higher the speed and resolution with which the switching events of the transistors proceed. Particularly when bipolar switching transistors are used, it is especially difficult to achieve brief switching events. Although the invention is applicable to circuits with field effect transistors, the problems and their solution according to the invention will be described below taking bipolar transistors as an example.
The problems the invention seeks to solve will first be presented with reference to FIG. 1a. FIG. 1a shows the standard situation, known from the prior art, of a power switching transistor T1, in this case a bipolar transistor, to which a free-wheeling diode D1 is connected anti-parallel. The three externally accessible terminals of the transistor are marked C for collector (work electrode), B for base (control electrode), and E for emitter (reference electrode). Let UCE be the voltage that drops between the collector and the emitter, UBE be the voltage that drops between the base and the emitter, IC be the current flowing into the collector, and IB be the current flowing into the base. Between the collector and the base, there is a parasitic capacitance, the so-called Miller capacitance CCB. This is correspondingly true for field effect transistors as well. The definitive power loss during a switching event is determined by the product ICxc2x7UCE. In FIG. 1b, the course of not only these two variables but also the course of the base-to-emitter voltage UBE and of the base current IB are plotted over time. A relatively loss-free and thus desirable switching event is characterized in that as soon as UCE begins to rise, the collector current IC drops sharply. The course over time of the variables IC and UCE, shown for the circuit in FIGS. 1a and 1b, does not meet this condition, for the following reasons: First, for the discussion below it will be assumed that the emitter of the transistor T1 is connected to ground. Thus if the voltage at the collector increases, that is, if UCE rises, then because of the charge current through the Miller capacitance CCB and because of the voltage-related xe2x80x9csoftnessxe2x80x9d of the base terminal, the base voltage UBE is xe2x80x9cslavedxe2x80x9d; that is, UBE also increases (see the circle in FIG. 1b) instead of decreasing. Since the base triggering has a so-called current source characteristic, the current that charges the Miller capacitance CCB acts with negative feedback on the base electrode of the transistor T1. The switch-off event is slowed down sharply as a result: While UCE is already rising, IC is still present in virtually its full intensity. This also becomes clear from the convex course of the curve of the current IC during the switch-off event. The shaded area marked A corresponds to the charge that still flows through the transistor even though the transistor has already been xe2x80x9cswitched offxe2x80x9d. The course of the current IL through an ohmic-inductive load, which is connected to the work electrode of the main switch, is indicated by dashed lines. The charge for the charge reversal event, marked by the area B, is accordingly furnished by transistor capacitors (not shown). The ratio of area A to area B, that is, of the switch-off current to the charge reversal current, can be considered a measure for the losses of the switchover event. The lower the ratio of A to B is, the less are the losses that occur in a switchover event.
Previous ways of speeding up such switch-off events provided on the one hand connecting a resistor, on an order of magnitude of less than 100xcexa9, parallel to the base-to-emitter path, and on the other connecting a series circuit comprising a capacitor with a capacitance of xe2x89xa610 nF and a be resistance of xe2x89xa6100xcexa9 parallel to the base-to-emitter path. The results of these versions are unsatisfactory, however, because the switch-off losses are still great, as they were before.
The object of the present invention is therefore to refine a circuit arrangement of the type defined at the outset such that the losses occurring in a switch-off event of the main switch are as slight as possible.
For attaining this object, the invention provides that each main switch is assigned an electric auxiliary switch, whose work electrode is connected to the control electrode of the associated main switch and whose reference electrode is connected to the reference electrode of the associated main switch, and at least one capacitor is disposed between the control electrode of the auxiliary switch and the work electrode of the associated main switch, and a discharge unit is disposed between the control electrode and the reference electrode of the auxiliary switch in such a way that the at least one capacitor can be discharged during the transition of the main switch from the OFF state to the ON state.
The embodiment according to the invention makes use of the recognition that a voltage increase at the work electrode of the main switch can be transmitted to the control electrode of the auxiliary switch via a capacitor. Given a suitable choice of the auxiliary switch, the charging event of the capacitor trips the transition of the switch to the ON state. Since the work electrode of the auxiliary switch is connected to the control electrode of the main switch, active charge carriers are thereby drawn from the control electrode of the main switch. The influence of the Miller capacitance CCB, which slows down the switch-off event of the main switch, can be counteracted as a result. This leads to a marked speeding up of the switch-off event and thus to markedly lesser losses than in the versions of the prior art. The discharge unit serves to discharge the capacitor between the switch-off events, so that it is again available for the ensuing switch-off event or in other words can be charged again.
In its simplest embodiment, the discharge unit comprises a single resistor. In an alternative embodiment, the discharge unit can include a discharge diode, preferably in the form of a Schottky diode, Zener diode or p-n diode; at least one resistor can be connected in series with and/or parallel to the discharge diode. The discharge unit can have a switch-off input terminal, by way of which a signal that switches the auxiliary switch to the ON state can be applied. The connection of the switch-off input terminal to the control electrode of the auxiliary switch preferably includes a shut-down diode, which with respect to the control electrode of the auxiliary switch is oriented like the discharge diode. The switch-off diode can be embodied as a p-n or Schottky diode.
The discharge unit can have a sense output terminal, which is connected to the control electrode of the auxiliary switch such that the switching status of the main switch and/or the auxiliary switch can be interrogated via the sense output terminal. At least one capacitor for shifting the potential can be disposed between the sense output terminal and the control electrode of the auxiliary switch.
The circuit arrangement of the invention can also have an antisaturation unit, which is disposed between the work electrode and the control electrode of the auxiliary switch and which prevents saturation of the control electrode of the auxiliary switch. In its simplest embodiment, the antisaturation unit can be embodied by a single resistor, which is disposed such that in the event of saturation of the control electrode of the auxiliary switch, charge carriers can flow away to the work electrode of the auxiliary switch. In an alternative, especially preferred embodiment, the antisaturation unit can have an antisaturation diode, which is disposed such that it performs the same purpose. The antisaturation diode can be a p-n or Schottky diode; at least one resistor can be connected in series with and/or parallel to the antisaturation diode.
A main switch can be a bipolar transistor, but it can also be realized with the associated free-wheeling diode by a MOSFET. As the auxiliary switch, the following can be considered: a bipolar transistor, a bipolar transistor with a series diode in the collector, a MOSFET, or a MOSFET with a series diode in the drain, and if a series diode is used, its orientation is made such that a current flow in the main flow direction of the associated auxiliary switch is made possible. This series diode is preferably in the form of a Schottky diode.
The invention also includes a bridge circuit having at least one first and one second circuit arrangement, in which a bridge is formed by connecting the reference electrode of the main switch of the second (xe2x80x9cupperxe2x80x9d) circuit arrangement to the work electrode of the main switch of the first (xe2x80x9clowerxe2x80x9d) circuit arrangement, optionally with the interposition of additional components. A bridge circuit of this kind can be part of a free-running oscillator circuit for operating a load, which furthermore has a switching control device for feedback of the load current to the control electrodes of the main switches of the first and second circuit arrangements, and the control electrode of each main switch is connected by a respective terminal line to the switching control device, and the two terminal lines are connected in turn to one another via at least one so-called base bridge capacitor. The switching control device can be a control transformer. The base bridge capacitor can be connected directly to the control electrode of each main switch and/or directly to the output of the switching control device oriented toward the control electrode of the applicable main switch.
Other advantageous refinements can be learned from the dependent claims.
Exemplary embodiments will be described in further detail below in conjunction with the accompanying drawings.